The feline oncornavirus-associated cell membrane antigen (FOCMA) is expressed on feline sarcoma virus (FeSV) transformed nonproducer (NP) fibroblasts as a polyprotein which also contains the feline leukemia virus (FeLV) gag gene peptides p15 and p12. The same antigen is expressed on cat virus producer and NP lymphoma cells as a 65,000-70,000 dalton molecule containing only FOCMA. This antigen is not on FeLV-infected normal cells. The FOCMA molecule is an integral cell membrane protein that is apparently non-glycosylated and it lacks protein kinase activity. Little or no FOCMA activity is found in the cytoplasm or nucleus. FOCMA as expressed on FeSV-transformed NP fibroblasts shares one major cross-reactive antigenic determinant with FOCMA on lymphoid tumor cells. An additional antigenic determinant was found on the FOCMA induced by FeSV (FOCMA-S) that is not on the FOCMA found on the lymphoid tumor cells (FOCMA-L). A significant proportion of cats that were naturally exposed to FeLV also have antibodies to the FOCMA-S specific antigen.